1. Field of the Invention
The present invention is directed to a radio system having multiple transmitters and multiple receivers and, more particularly, to an antenna interface unit for connecting receivers and transmitters to an antenna while providing low power loss, fault-tolerant connectivity.
2. Description of the Related Art
There are many applications where multiple radio frequency (RF) transceivers (each including a transmitter and a receiver) are required in a fixed or a mobile station. Such transceivers are used, for example, in commercial airplanes, military airplanes, space platforms, military manpacks, and communications hubs. Each transceiver typically performs a single communication, navigation, surveillance or telemetry (CNST) function. Moreover, critical functions (such as avionics, instrument landing systems or communication links) require redundant transceivers to ensure high availability. These redundant transceivers must possess fault-tolerant RF connectivity to corresponding antennas. That is, the transceivers must be connected to an antenna such that a single-point failure (or sometimes a multiple-point failure) in the RF chain between the antenna and the transceivers does not cause the loss of a communication, navigation, surveillance or telemetry function. Similarly, where multiple antennas are used, the transceivers must be connected to the antennas such that a single-point or sometimes multiple-point failure in the RF chain between the antennas and the transceivers does not cause the loss of a communication, navigation, surveillance or telemetry function.
FIG. 1 illustrates a conventional system in which a main transceiver 20 and redundant transceivers 22 and 24 are connected to a single antenna 28 through a single pole multiple throw switch (SPMT) 26. A receive channel extends from antenna 28, through switch 26 and to a selected transceiver 20, 22 or 24. A transmit channel extends from a selected transceiver 20, 22 or 24, through switch 26 and to antenna 28. This system is not fault-tolerant because a single-point failure in switch 26 could deny antenna connectivity to all transceivers 20, 22 and 24, thereby breaking the transmit and receive channels.
FIG. 2 illustrates the use of a conventional RF power combiner/splitter 30, instead of switch 26. Combiner/splitter 30 is made of passive components and has essentially a zero failure rate. Thus, combiner/splitter 30 is an acceptable device for achieving the desired connectivity without the chance of a single-point failure. When it is desired to transmit information, combiner/splitter 30 operates as an RF power combiner by combining signals from transceivers 20, 22 and 24 into a combined signal which is provided to antenna 28. Therefore, the transmit channel extends from transceivers 20, 22 and 24, to RF power combiner/splitter 30 to antenna 28. Usually, however, only one of transceivers 20, 22 and 24 is turned ON to transmit at any time, and the other two transceivers act as redundant spares.
When it is desired to receive information, combiner/splitter 30 receives a signal from antenna 28 and operates as an RF power splitter by splitting the signal into three separate signals which are respectively provided to transceivers 20, 22 and 24. Therefore, the receive channel extends from antenna 28, to RF power combiner/splitter 30 to transceivers 20, 22 and 24. Both the transmit channel and the receive channel follow the same path and are essentially the same channel. That is, the same channel, extending from transceivers 20, 22 and 24 to antenna 28, operates as both the transmit channel and the receive channel. In both the transmit and receive channels, each transceiver 20, 22 and 24 is connected to antenna 28 via combiner/splitter 30. This can be compared to FIG. 1, where only one transceiver 20, 22 or 24 is connected to antenna 28 at a particular time.
Unfortunately, an RF power combiner/splitter (such as combiner/splitter 30) adds unacceptable loss in both the receive and transmit channels. For example, there is approximately a 5 Db loss for a conventional three-way combiner/splitter. Although some communication, navigation, surveillance or telemetry functions can afford such an additional front-end loss in the receive channel, the use of a three-way combiner requires that each transceiver 20, 22 and 24 is designed to supply RF transmit power that is three times higher than the amount needed if only one transceiver is connected directly to antenna 28. As a general example, if it is desired to produce a 40 watt signal at antenna 28 in the transmit channel, each transceiver 20, 22 and 24 in FIG. 1 should provide a 40 watt signal to switch 26. However, with the system in FIG. 2, each transceiver 20, 22 and 24 should provide approximately a 120 watt signal to combiner/splitter 30 to produce a 40 watt signal at antenna 28 due to the power loss in combiner/splitter 30. Thus, power combining imposes a severe power dissipation penalty for communication, navigation, surveillance or telemetry functions. This power dissipation penalty is especially high at high power levels. Similarly, in the receive channel, the signals received by each transceiver 20, 22 and 24 each have approximately 1/3 the power of the signal received by antenna 28. An amplifier (not illustrated) can be positioned between combiner/splitter 30 and antenna 28 to amplify transmitted and received signals. Unfortunately, the amplifier would be the source of a single-point failure that could bring the entire system down.
FIG. 3 illustrates a system having redundant transceivers without having a potential single-point failure and without paying a power penalty. This type of system is particularly used in commercial air transports. As illustrated in FIG. 3, each transceiver 20, 22 and 24 is respectively coupled to a corresponding antenna 32, 34, 36, without the use of a switch or an RF power combiner/splitter. Although this use of separate antennas 32, 34 and 36 avoids high loss in the receive and transmit channels, and also avoids single-point failures associated with single-pole multiple throw switches, the use of separate antennas creates large, undesirable antenna farms.
Moreover, a programmable radio is capable of operating at many different radio functions (for example, VHF radio is one radio function). Therefore, it may be desirable to use programmable radios for each of transceivers 20, 22 and 24. However, if antennas 32, 34 and 36 are radio specific (that is, an antenna services only one or several specific radio functions), very little will be gained by using transceivers 20, 22 and 24 programmable to operate for many different radio functions, since the corresponding antennas would not be properly operable at the different radio functions.
FIG. 4 illustrates a conventional system which uses a conventional fault-tolerant power amplifier 40. In FIG. 4, a main transmitter 42 and redundant transmitters 44 and 46 are connected to an RF power combiner 48. RF power combiner 48 receives signals from transmitters 42, 44 and 46 and combines the signals to produce a combined signal on line 50. The combined signal on line 50 is fed to an RF power splitter 52 which splits the combined signal into a plurality of split signals which are respectively provided on parallel channels 54. Each parallel channel 54 includes amplifiers 56 to amplify the corresponding split signal. Split signals travelling along parallel channels 54 are then received and combined by an RF power combiner 58. RF power combiner 58 outputs a combined signal on line 60 which is fed to a transmit/receive switch (T/R SW) 62. T/R switch 62 is connected to a single antenna 64. Thus, the transmit channel extends from transmitters 42, 42 and 46, to RF power combiner 48, RF power splitter 52, RF power combiner 58 and to antenna 64. Fault-tolerant power amplifier 40 employs parallel channels 54 for redundancy, and parallel channels 54 are amplitude and phase matched between RF power splitter 52 and RF power combiner 58. Therefore, as illustrated by FIG. 4, fault-tolerant power amplifier 40 includes RF power combiner 58, RF power splitter 52 and parallel channels 54.
On the receive channel, a signal is received by antenna 64 and passes through T/R switch 62 to an RF power splitter 66. RF power splitter 66 is connected to a main receiver 68 and redundant receivers 70 and 72 via bandpass filters (BPF) 74, automatic gain controllers (AGC) 76, and low noise amplifiers (LNA) 78. Thus, the receive channel extends from antenna 64, to RF power splitter 66 and to receivers 68, 70 and 72. Redundant transmitters 44 and 46 can be connected to switches 80 and 82, respectively, for use with other communication, navigation, surveillance and telemetry functions.
While BPFs 74, AGCs 76 and LNAs 78 are illustrated in FIG. 4, other receiver front-end elements, such as receiver protectors (not illustrated), can be included between RF power splitter 66 and receivers 68, 70 and 72.
With a system as illustrated in FIG. 4, the receive and transmit channels are partitioned into separate channels. For example, signals travelling along the receive channel travel along a different route than signals travelling along the transmit channel. This partitioning or separation of the transmit channel and the receive channel avoids part of the single-point failure problem by using fault-tolerant power amplifier 40, and can be contrasted to FIG. 2 in which the same channel serves as both the transmit channel and the receive channel. Each single-point failure in one parallel channel 54 of fault-tolerant power amplifier 40 results only in the loss of a relatively small amount of transmit power, and sufficient power margin can be provided to allow a moderate number of failures before the total output power drops below the system requirement.
Transmitters 42, 44 and 46 are connected to fault-tolerant power amplifier 40 in a fault-tolerant manner by using RF power combiner 48 at the input to fault-tolerant power amplifier 40. The low-power RF loss resulting from RF power combiner 48 can be compensated with additional low-power gain in parallel channels 54. As a result, transmit power in the transmit channel is not lost, as compared to a system as illustrated in FIG. 2.
However, in the receive channel, RF power splitter 66 causes a front-end RF loss and causes a direct increase in the receive noise figure. The receive noise figure increases because the amount of noise in a received signal has a minimum level, called a noise "floor". The level of noise can not be reduced below the noise floor. Therefore, in RF power splitter 66, the signal power is reduced by approximately one-third, thereby resulting in approximately in a one-third reduction in the signal level. However, due to the noise floor, the noise signal is not reduced by one-third. Thus, the signal to noise ratio (S/N) of the received signal is less at the output of RF power splitter 66 than it was at the input to RF power splitter 66. As a result, the receive channel from RF power splitter 66 to receiver 68, the receive channel from RF power splitter 66 to receiver 70 and the receive channel from RF power splitter 66 to receiver 72 each experience a receive noise figure penalty due to the loss in the RF power splitter 66.
Moreover, the system as disclosed in FIG. 4 does not have fault-tolerance for the complete system because T/R switch 62 is still the source of a single-point failure. Thus, the failure of T/R switch 62 will bring the entire system down. The use of a circulator in place of T/R switch 62 may eliminate the source of a single-point failure in transmit/receive duplexing, but doing so assumes that a circulator will be acceptable as failure free and that practical applicators can be realized at the desire operating frequency. However, circulators are not practical for many important applications below 500 MHz.